I wouldn't feel unarmed with a BP revolver.

Back in the late 70's I bought a Ruger old army cap and ball revolver. It was very accurate with a moderate load of black powder. At 75 yards I could keep them in a picnic size paper plate off a sand bag. I don't recall what the charge I used was but I remember the groups really opened up when I started getting near max loads. I took a good size six point buck one year with that gun and that same medium load. I don't see any reason it wouldn't work just as good for defense.

Edited to add: If I were going to get a BP rifle I would get one of the newer inline rifles. I have never used one, but I know some guys that hunt exclusively with BP. They tell me the inline rifles are more accurate than a traditional BP rifle and that you can scope the inline rifles.